Green Bean Casserole with Onion Rings

Name: Green Bean Casserole with Onion Rings
Ingredients: 1 1/2 lb. green beans, halved crosswise; kosher salt; 7 tbsp. butter, divided; 1 medium yellow onion, sliced into rings; 8 oz. button mushrooms, trimmed and sliced; Freshly ground black pepper; 5 tbsp. all-purpose flour; 3 c. whole milk; 1/2 c. cornstarch; 1/2 tsp. baking powder; 1/4 tsp. baking soda; 1/4 tsp. cayenne pepper; 1 c. all-purpose flour, divided; 3/4 c. club soda, chilled; 1 medium onion, sliced into 1/2-thick rings; Vegetable oil for frying